Substitution Control is a web server control that provides partial page caching.It is related with output caching and is used to define a section on the output cached page where dynamic contents can be substituted for the control. In other words we can say that the substitution control will not be cached even though it's on the output cached web page.
The AdRotator control directly supports post-cache substitution and renders a new advertisement every time that the page posts back, regardless of whether the page is cached
< asp:substitution id="Substitution1" methodname="GetCurrentDateTime" runat="Server">
< / asp:substitution >